1. Field of the Invention
The invention relates to liquid crystal picture screen provided with a backlight system which comprises at least one gas discharge lamp with a discharge vessel and at least one capacitive coupling means with a dielectric material. The invention further relates to a backlight system with at least one gas discharge lamp and to a gas discharge lamp.
2. Description of Related Art
Liquid crystal picture screens are passive display systems, i.e. they do not themselves radiate light. These picture screens are based on the principle that light does or does not pass through the layer of liquid crystals. This means that an external light source is required for generating a picture. The ambient light is utilized as an external light source in reflective liquid crystal picture screens. In the case of transmissive liquid crystal picture screens, artificial light is generated in a backlight system.
Backlight systems may comprise a gas discharge lamp as the light source. Besides the generation of electrons at so-called hot electrodes through glow emission, the gas discharge may also be caused by emission of electrons in a strong electric field or directly through ion bombardment (ion-induced secondary emission). In a capacitive mode of operation, capacitive coupling means are used as the electrodes. These capacitive coupling means are formed from a dielectric material which is in contact with the discharge gas at one side and which is connected with electrical conduction to an external current circuit at the other side. An AC voltage applied to the capacitive coupling means generates an electric AC field in the discharge vessel in which the electrons move and excite a gas discharge in a known manner.
Such a gas discharge lamp is known from DE 199 15 616, in which a sintered ferroelectric material is used as the coupling means instead of the usual metal electrodes. The sintered ferroelectric material used was preferably Ba(Ti0.91Zr0.09)O3 doped with certain donor/acceptor combinations. The material described has a high dielectric constant and a rectangular hysteresis loop.
It is a disadvantage of the materials used that they are coarse-grained. As a consequence the capacitive coupling means has a reduced mechanical strength. A further disadvantage is that the Curie temperature of the ferroelectric material is very low, at 80xc2x0 C., so that the operating temperature of the lamp is very low.
It is accordingly an object of the invention to circumvent the disadvantages of the present state of the art and to provide a liquid crystal picture screen with an improved backlighting, in particular with an improved light source.
This object is achieved by means of a liquid crystal picture screen provided with a backlight system which comprises at least one gas discharge lamp with a discharge vessel and at least one capacitive coupling means with a dielectric material with the composition [Axe2x80x2a1n1+Axe2x80x3a2n2+ . . . Anxe2x80x2annn+] [Bxe2x80x2b1m1+Bxe2x80x3b2m2+ . . . Bmxe2x80x2bmmn+]O3, wherein the cations
Axe2x80x2a1n1+Axe2x80x3a2n2+ . . . Anxe2x80x2annn+ comprise at least one or several of the cations chosen from the group of Ba2+, Pb2+, Sr2+ and Ca2+, as well as possibly one or several of the cations chosen from the group of Cs1+, Rb1+, Tl1+, K1+, Pb2+, Ag1+, Sr2+, Na1+, Bi3+, La3+, Mg2+, Zn2+, Ca2+, Ce3+, Cd2+, Pr3+, Nd3+, Eu3+, Gd3+ and Sm3+, and
the cations Bxe2x80x2b1m1+Bxe2x80x3b2m2+ . . . Bmxe2x80x2bmmn+ comprise at least one or several of the cations chosen from the group of Ti4+, Zr4+ and Sn4+ as well as possibly one or several of the cations chosen from the group of Mn2+, Cr2+, In3+, V2+, Fe2+, Pb4+, Li1+, Co2+, Sc3+, Zn2+, Cu2+, U6+, Mg2+, Hf4+, Mo3+, Ni2+, Nb4+, Ti3+, W4+, Mo4+, Fe3+, Mn3+, V3+, Re4+, Ir4+, Ru4+, W5+, Ta5+, Cr3+, Ga3+, Co3+, Mo5+, Ni3+, Sb5+, W6+, Nb5+, Mo6+, Fe4+, Re5+, V4+, Te6+, V5+, Cu3+, Al3+, Mn4+, Ge4+, Y3+, Gd3+, Dy3+, Ho3+, Er3+, Yb3+, Tb3+ and Lu3+, with
0.98xe2x89xa6a1+a2+ . . . +anxe2x89xa61.02,
0.98xe2x89xa6b1+b2+ . . . +bmxe2x89xa61.02,
a1+a2+ . . . +an+b1+b2+ . . . +bmxe2x89xa62,
a1*n1+a2*n2+ . . . +an*nn+b1*m1+b2*m2+ . . . +bm*mmxe2x89xa66.
A liquid crystal picture screen with improved characteristics is obtained through the use of the dielectric materials according to the invention in the capacitive coupling means of the gas discharge lamp. The materials according to the invention can be manufactured with comparatively small dimensions (xe2x89xa620 xcexcm). The capacitive coupling means comprising a dielectric material according to the invention as a result has a greater mechanical strength and is accordingly clearly more durable. The improved breakdown resistance renders it possible to manufacture a capacitive coupling means with a smaller layer thickness. A further advantage is that the dielectric materials show a smaller cathode fall, i.e. lower losses in the coupling of the current into the gas discharge lamp, in comparison with the prior art mentioned above. All in all, a more efficient gas discharge lamp is obtained thereby. A further advantage is that the dielectric materials according to the invention join themselves more securely to glass. This enhances the vacuumtightness of the gas discharge lamp. Furthermore, the dielectric materials according to the invention have a Curie temperature of more than 80xc2x0 C.
It is preferred that the cations Axe2x80x2a1n1+Axe2x80x3a2n2+ . . . Anxe2x80x2annn+ comprise Ba2+ and the cations Bxe2x80x2b1m1+Bxe2x80x3b2m2+ . . . Bmxe2x80x2bmmn+ comprise Nb5+, Co2+ and Mn3+.
A capacitive coupling means with this dielectric material distinguishes itself by a high strength, a low cathode fall, and a Curie temperature of 125xc2x0 C.
It may furthermore be preferred that the capacitive coupling means in addition comprises a sintering aid.
It is particularly highly preferred that said sintering aid is SiO2.
The manufacture of the capacitive coupling means is improved by sintering aids. The sintering aid may in addition have a positive influence on the properties of the dielectric material and thus of the coupling means.
It is particularly highly preferred that the cations Axe2x80x2a1n1+Axe2x80x3a2n2+ . . . Anxe2x80x2annn+ comprise Ba2+ and Mg2+, the cations Bxe2x80x2b1m1+Bxe2x80x3b2m2+ . . . Bmxe2x80x2bmmn+ comprise Y3+, W6+, Mo6+ and Mn2+, and the additional compound is SiO2.
A capacitive coupling means with this dielectric material distinguishes itself by a high strength, a low cathode fall, and a Curie temperature of 125xc2x0 C.
The invention further relates to a backlight system which comprises at least one gas discharge lamp as well as to a gas discharge lamp with a discharge vessel and at least one capacitive coupling means with a dielectric material having the composition:
[Axe2x80x2a1n1+Axe2x80x3a2n2+ . . . Anxe2x80x2annn+] [Bxe2x80x2b1m1+Bxe2x80x3b2m2+ . . . Bmxe2x80x2bmmn+]O3, wherein the cations Axe2x80x2a1n1+Axe2x80x3a2n2+ . . . Anxe2x80x2annn+ comprise at least one or several of the cations chosen from the group of Ba2+, Pb2+, Sr2+ and Ca2+, as well as possibly one or several of the cations chosen from the group of Cs1+, Rb1+, Tl1+, K1+, Pb2+, Ag1+, Sr2+, Na1+, Bi3+, La3+, Mg2+, Zn2+, Ca2+, Ce2+, Cd2+, Pr3+, Nd3+, Eu3+, Gd3+ and Sm3+, and
the cations Bxe2x80x2b1m1+Bxe2x80x3b2m2+ . . . Bmxe2x80x2bmmn+ comprise at least one or several of the cations chosen from the group of Ti4+, Zr4+ and Sn4+ as well as possibly one or several of the cations chosen from the group of Mn2+, Cr2+, In3+, V2+, Fe2+, Pb4+, Li1+, Co2+, Sc3+, Zn2+, Cu2+, U6+, Mg2+, Hf4+, Mo3+, Ni2+, Nb4+, Ti3+, W4+, Mo4+, Fe3+, Mn3+, V3+, Re4+, Ir4+, Ru4+, W5+, Ta5+, Cr3+, Ga3+, Co3+, Mo5+, Ni3+, Sb5+, W6+, Nb5+, Mo6+, Fe4+, Re5+, V4+, Te6+, V5+, Cu3+, Al3+, Mn4+, Ge4+, Y3+, Gd3+, Dy3+, Ho3+, Er3+, Yb3+, Tb3+ and Lu3+, with
0.98xe2x89xa6a1+a2+ . . . +anxe2x89xa61.02,
0.98xe2x89xa6b1+b2+ . . . +bmxe2x89xa61.02,
a1+a2+ . . . +an+b1+b2+ . . . +bmxe2x89xa62,
a1*n1+a2*n2+ . . . +an*nn+b1*m1+b2*m2+ . . . +bm*mmxe2x89xa66.